cgen: route integer interpolation through str_intp

[medvednikov]

Fixes #27574

Summary:

• Route primitive integer simple interpolation through str_intp outside builtin internals, avoiding the int_str + string_plus_many lowering.
• Keep markused in sync for direct and generic string interpolation so str_intp runtime helpers stay available when needed.
• Preserve .str()/auto-str helper dependencies for s-formatted operands when another operand forces runtime interpolation.
• Leave untyped integer literals on the existing stringification path, preserving large literal values such as ${2147483648}.
• Keep aggregate smartcast interpolation with auto s formatting on the existing simple stringification path.
• Add regressions for the os + -skip-unused helper retention case, mixed integer plus auto-str operands, the large integer literal path, and grouped aggregate smartcasts.

Tests:

• ./v -g -keepc -o ./vnew cmd/v
• ./vnew -silent vlib/v/compiler_errors_test.v
• ./vnew -silent test -run-only test_skip_unused_runtime_integer_interpolation_marks_auto_str_operand vlib/v/gen/c/coutput_test.v
• ./vnew -silent test -run-only test_skip_unused_aggregate_smartcast_integer_interpolation_uses_simple_path vlib/v/gen/c/coutput_test.v
• ./vnew -silent test -run-only test_skip_unused_os_integer_interpolation_keeps_str_intp_runtime vlib/v/gen/c/coutput_test.v
• ./vnew -silent test -run-only test_c_must_have_files vlib/v/gen/c/coutput_test.v
• ./vnew -skip-unused run vlib/v/gen/c/testdata/string_interpolation_simple_optimization.vv
• ./vnew run vlib/v/checker/tests/run/comptime_ident_is_type.vv
• ./vnew -silent test vlib/v/tests/builtin_strings_and_interpolation/

Notes:

• A full ./vnew -silent vlib/v/gen/c/coutput_test.v run still hit an unrelated translated_module.vv mismatch with empty output instead of result: 104.
• A broad ./vnew -silent test vlib/v/ run was stopped after unrelated failures from an untracked cmd/v2/_tmp_map_empty_raw.v parser fixture pickup and tcc fallback warning output pollution.
• An attempted parallel coutput validation run failed because coutput_test.v shares temp directories and process cwd state across tests; the relevant coutput checks were rerun sequentially with test -run-only and passed.

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